Effective selection criteria for assessing the resistance of stink bugs complex in soybean

Critério efetivo de seleção para avaliar a resistência ao complexo de percevejos em soja

ARTICLE

Effective selection criteria for assessing the resistance of stink bugs complex in soybean

Critério efetivo de seleção para avaliar a resistência ao complexo de percevejos em soja

Fabiani da Rocha; Felipe Bermudez; Mônica Christina Ferreira; Kênia Carvalho de Oliveira; José Baldin Pinheiro^*

Universidade de São Paulo, Escola Superior deAgricultura “Luiz de Queiroz”, Departamento de Genética, Avenida Pádua Dias, 11, 13.418-900, Piracicaba, SP, Brazil

ABSTRACT

Soybean plants with resistance to the stink bug complex are currently selected by extremely labor-intensive methods, which limit the evaluation of a large number of genotypes. Thus, this paper proposed the use of an alternative trait underlying the selection of resistant genotypes under field conditions with natural infestation: the weightof healthy seeds (WHS). To this end, 24 genotypes were evaluated under two management systems: with systematic chemical control of insects (management I), and without control (management II). Different indices were calculated using grain weight (Y_P) of management I and WHS ofmanagement II (Y_S). The high correlation between Y_Sand the indices mean productivity, stress tolerance and geometric mean productivity, plus the agreement in determining the groups of genotypes with resistance and high yield indicate that WHS is a useful character insimultaneous selection for these traits.

Key words: Glycine max, Nezara viridula, Piezodorus guildinii, Euschistus heros

RESUMO

A seleção de plantas em soja com resistência ao complexo de percevejos é feitaatualmente com base em métodos laboriosos, o que limita a avaliação de um grandenúmero de genótipos. Assim, este trabalho propôs uma nova alternativa para aseleção de genótipos resistentes, em condição de campo com infestação natural:a massa de sementes boas (MSB). Para isto, 24 genótipos de soja foram avaliadossob dois sistemas de manejo: com controle químico sistemático de insetos(manejo I), e sem nenhum controle (manejo II). Diferentes índices foramestimados utilizando a massa de grãos (Y_P) do manejo I e o MSB do manejo II (Y_S). A altacorrelação entre Y_S e os índices de produtividade média, tolerânciaao estresse e média geométrica, aliada à concordância na determinação dosgrupos de genótipos com resistência e alto rendimento indicam que a MSB é um caráter útil na seleção simultânea para estas características.

Palavras-chave: Glycine max, Nezara viridula, Piezodorus guildinii, Euschistus heros

INTRODUCTION

Soybean is a legume of great world wide importance, however its production can be affected by a number of both bioticand abiotic stresses. In this scenario, insect pests are influential, causingboth direct (when attacking the marketable plant parts) and indirect damage tocrops, and may also act on pathogen transmission (Gallo et al. 2002).

Phytophagous pentatomids (sucking bugs) areamong the main pests of soybean (Godoi and Pinheiro 2009, Guedes et al. 2012).Known as stink bug complex, the species Nezara viridula (L.), Piezodorusguildinii (West.) and Euschistus heros (Fabr.) attack mainly duringpod formation and maturation (Panizzi and Slansky Junior 1985, Godoi et al.2002). The damage is caused basically by larger nymphs, from the 3^rdto the 5^th instars, and adults that feed directly on soybean seeds, piercing the pods and extracting nutrients from the seed with theirpiercing-sucking mouthpart (McPherson and McPherson 2000), resulting in lossesin grain yield and quality. Plant maturation can also be delayed when the seedsare significantly injured (Leonard et al. 2011).

As a means to mitigate the effectsof these insect pests on crops, insecticides have been intensively applied.However, this control method is harmful to the environment, leaving waste andpromoting the selection of resistant populations (Maia et al. 2009). In 2000, decreased susceptibility of Euschistus heros (Fabr.) to the insecticidemethamidophos (Sosa-Gómez et al 2001) was found in the State of São Paulo; andmore recently a higher number of resistant genotypes was observed in the Stateof Paraná (Sosa-Gómez and Silva 2010). Moreover, there is a trend in thecurrent Brazilian scenario to reduce the number of active ingredients availablefor stink bug control, prohibition of some organophosphate insecticides inaddition to the lack of innovation and introduction of new insecticides (Guedeset al. 2012).

Thus, the development of soybean cultivarsresistant to the stink bug complex is extremely meaningful for the maintenanceand/or increase in yield levels of this crop. However, current strategies, suchas the percentage index of pod damage (Rossetto et al. 1986, Nagai et al. 1987)and percentage of spotted seeds (Hoffmann-Campo et al. 1988) for selection ofresistant genotypes are extremely labor-intensive, which limits the evaluationof a large number of genotypes without ensuring the selection of thehighest-yielding. Thus, the objective of this study was to show, based onresistance indices, that the weight of healthy seeds can be used as analternative trait for the selection of soybean genotypes resistant to the stink bug complex and with high yieldpotential.

MATERIAL AND METHODS

The study was carried out in the 2011/12growing season in Piracicaba, São Paulo, at the Experimental Station Anhumas.The reaction resistance of 24 genotypes (two of which are transgenic cultivars)to the stink bug complex was evaluated in two experiments, in a randomizedblock design with five replications under two management systems: with chemicalinsect control (management I), and without control measures (management II).Among the genotypes, cultivar IAC 100, developed by the Agronomic Institute ofCampinas - IAC, is known to be moderately resistant to sucking (stink bugcomplex) and chewing insects (caterpillars and beetles) (Veiga et al. 1999).Other lines included in this research, denominated “LQ” (lines of soybean plantbreeding program at the “Luiz de Queiroz” College of Agriculture), are alsoresistant to stink bug.

In management I, systematic and preventivespraying was applied five times while no insect control measures were used inmanagement II. The experimental plot consisted of four 5-m long rows, spaced0.5 m apart. The insects were sampled in the experimental area by a drop cloth(two meters of a row), with 20 droppings per experiment and daily assessment.The characters grain yield (GY) and weight of healthy seeds (WHS) wereevaluated, both in kg plot^-1. The WHS was determined by discardingempty, green and malformed grains, with the use of a spiral, in which the seedsare separated by the action of gravity and centrifugal forces.

To evaluate the resistance of the differentgenotypes to the stink bug complex, indices were used, based on the GY ofmanagement I representing the potential yield of the genotype (Y_P), i.e., yield in the absence of stress (no stink bug damage), and WHS ofmanagement II, representing the yield of genotypes under stress (Y_S).

The resistance indices of genotypes to thestink bug complex were estimated by the equations proposed by Fernandez (1992):

Stress susceptibility index (SSI):

Tolerance (TOL): TOL_i = Y_Pi – Y_Si

Mean productivity (MP):

Stress tolerance index (STI):

Geometricmean productivity (GMP):

where Y_Si represents the yield of the i^th genotype under stress (WHSmanagement II), Y_Pi the yield potential of the i^thgenotype in the absence of stress (GY under management I), Y_S and Y_P the genotype means in both environments, withand without stress, respectively.

Analyses of individual variance, F-test andPearson correlation for the traits Y_S, Y_P and for the indices estimated from these were performed. When the F testdetected significant differences, tests of treatment means (Scott-Knottprobability level of 0.05) were performed.

RESULTS AND DISCUSSION

As expected in theexperiment without chemical insect control, there was a significant increase inthe population of stink bugs at the end of the crop cycle (Figure 1). This canbe explained by the presence of pods on the plants, which is directly relatedto the presence of stink bugs in the crop, and insect migration from alreadyharvested neighboring areas (Panizzi et al. 2000). The average number of bugsranged from 0 to 1 in the experiment with chemical insect control (ManagementI), and reached 13 in management II. According to Corrêa-Ferreira and Panizzi(1999), the control should be performed when the population reaches four bugs(nymphs of the third instar or adults) per drop cloth (in two meters of a row).Thus, stink bug infestation in management II was high enough to evaluate the reactionof genotypes, allowing a discrimination among them.

The analysis of variance for grain yield potential (Y_Pi), weight of healthy seeds (Y_Si)and the indices estimated from these (Table 1) demonstrates the variabilityamong genotypes, allowing an identification of those with ability to supportstink bug attack. The coefficients of variation (CV) ranged from 19.88 to51.27%. High CV values can be explainedbecause some genotypes are more affected than others by the stink bug attack.

Among all evaluated genotypes, BRS 133, BRSInvernada, LQ1043, LQ1194, LQ1413, LQ1421, LQ1505, JAB 00-05-6/763D, and JAB00-02-2/2J3D had the highest yields in the environment without stress(management I) (Table 2), whereas in the environment with stress (managementII) the genotypes BRS 133, LQ1050, LQ1421 and LQ1505 had the highest Y_S values. These results show that selection for GY cannot ensure the identificationof those with higher resistance. This is the case of BRS Invernada, LQ1043, LQ1194, LQ1413, JAB 00-05-6/763D, and JAB 00-02-2/2J3D, which despite the highyield potential, were extremely stress-sensitive, with reduction in Y_S values (Table 2).

Considering the SSI index, three groups wereformed. For this index, LQ1050 was classified as the most resistant genotype tothe stink bug complex (Table 2). By the TOL index however, apart from LQ1050, genotypes BRS 133, LQ1421, LQ1505, LQ1402, LQ1504, IAC 100, LQ1519, L1-1-55, IAC 23, LQ1078, IAC 17, and BMX Potência RR had higher resistance to stink bugsthan the others. Soybean cultivar IAC 100 was characterized as resistant to thestink bug complex, based on at least five mechanisms: shorter grain filling period, more seeds, abscission of damaged pods and replacement by regrowth, normalsenescence with leaf fall at maturity and resistance to yeast Eremotheciumcoryli (Peglion) (Rossetto et al. 1995). The evaluated LQ lines werederived from IAC 100, and therefore expected to be grouped together.

However, both indexes, SSI and TOL, may favorthe selection of genotypes with high values under stress conditions (Y_S)but with low yield potential in the absence of stress (Fernandes 1992). Thiswas the case with LQ1050, which was classified as resistant by these twoindices, but is not in the group of highest Y_P values. Theseindices consider a proportion (SSI) or a difference (TOL) between the Y_S and Y_P values. Therefore, the smaller the amplitude of thesevalues, the stronger the genotype resistance. Thus, if a genotype islow-yielding in the absence of stress and maintains these levels under stress, the difference between yields will be reduced and the genotype is consideredresistant.

By index MP, the genotypes BRS 133, LQ1421, LQ1050, LQ1413, LQ1421, and LQ1505 performed best. By the indices STI and GMP, the genotypes BRS 133, LQ1421, and LQ1505 performed extremely well. Selectionbased on these indices favors genotypes with high yield under both managementconditions (Abarshahr et al. 2011). However, GMP is less sensitive to extremevalues (widely discrepant Y_S and Y_P), making this index more suitable for the distinction of genotype groups(Fernandes 1992). According to Talebi et al. (2009), the correlation betweenGMP and STI is approximately one, as similarly found in this study (0.98). Thisexplains why the groupings by both indices were the same.

High and significant correlations were found, considering Y_S and the indices (Table 3). Correlations of de Y_S with SSI and TOL were negative, since higher Y_S indicategenotypes with greater ability to withstand the insect attack, while for TOLand SSI higher values indicate greater susceptibility of a genotype.

Conversely, correlations of Y_S with MP, STI and GMP were positive, since higher values of Y_S and theseindices indicate increased resistance of the genotype (Table 3). The highcorrelations of Y_S with these indices show that Y_S can be used as an effective parameter in the selection of genotypes withresistance to the stink bug complex and high-yielding in the presence orabsence of stress. The selection of genotypes with this performance isadmittedly a challenge for plant breeders, while the yield in favorableenvironments was successfully increased (Richards et al. 2002).

The selected genotypes based on Y_S or MP, SSI and GMP indices agree, with some exceptions. Y_S iseasy to estimate under stress, thus the selection based on this parameterfacilitates the assessment for a high number of genotypes, allowing an increasein the number of repetitions and experimental locations, with no need fortesting different management types (with and without insect control). Theresults also indicate that WHS is a useful character in simultaneous selectionfor high yield and resistance to the stink bug complex. This fact is highlyrelevant, since farmers will only accept a new resistant cultivar if it is alsohigh-yielding in the presence or absence of stink bug attack. Moreover, plantresistance is a very promising strategy because it generates no adoption costwhile being compatible with the other forms of insect control.

ACKNOWLEDGEMENTS

The authors thank the National Council for Scientific and Technological Development (CNPq), Brazilian Federal Agency for Support and Evaluation of Graduate Education (CAPES) and São Paulo Research Foundation (FAPESP) for granting a scholarship and funding this study.

Received 22 February 2013

Accepted 07 June 2014

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Publication Dates

• Publication in this collection
  25 Nov 2014
• Date of issue
  Oct 2014

History

• Received
  22 Feb 2013
• Accepted
  07 June 2014